I have uploaded the custom add new form into SharePoint list. and I have tried the below PowerShell script for set as default add new form .
$listname = "List1"
$web = Get-SPWeb http://SiteUrl
$list = $web.lists[$listname]
$list.DefaultNewFormUrl="SiteUrl/ListName/NewForm.aspx"
but I got error :- Exception setting "DefaultNewFormUrl": "Unable to find an SPForm matching URL"